text:
Can you solve this fuel consumption equation?

Nasair 300, radar contact at 2205z, maintain flight level 300 report
  36 nm from present position.

If the ground speed is 360 knots, and fuel flow is 2400 kg/hr; what approximate time will Nasair 300 flight report 36 nm and how much approximate fuel will it burn from its present position to 36 nm, respectively?

This is arithmetic. Is this your homework?

Nope, it's a prep question for a test I'm about to take that I couldn't figure out.

homework or prep question does not make a big of a difference for us, we still prefer to know why you could not answer that question by yourself, so to give an answer that will help you understand better, instead of an answer that says "it's like this"

I have the answer in hand before posting the question I wanted to know how it came out to 240 kg and 6 min. I don't see why you making a big deal about it, this is what the website for sharing knowledge and helping each other.

Because that's the way this website works, we do not like questions that ask "please solve this equation" without explaining why you're not able to solve it yourself.

I'm voting to close this question as off-topic because OP is just asking for someone to do his study homework.

Hello Abdull. The issue that people seem to have is not so much about the question itself, but the way it is asked. If you would rephase the question in such a way that a) you explain the context (i.e. you are studying and you came accross this question), b) you explain what you have tried to solve it. c) you ask what the appropriate way is to calculate the answer to such a question, then I don't think anybody would object. The way people interpret your question now is: "here is my exam question, please give me the number so I can answer and pass my exam without understanding".

At $ v = 360 $ knots ground speed it will take one tenth of an hour to travel $ d = 36 $ NM:
$$ t = \frac{d}{v} = \frac{36 \, \mathrm{NM}}{360 \, \mathrm{NM} / \mathrm{h}} = 6 \, \mathrm{min}, $$
which means at 22:11Z. At a fuel flow of $ \dot{m} = 2400 $ kg/h, the plane will then burn 240 kg of fuel:
$$ m_\mathrm{Fuel} = t \times \dot{m} = \frac{1 \mathrm{h}}{10} \times 2400 \frac{\mathrm{kg}}{\mathrm{h}} = 240 \, \mathrm{kg} $$

next time, please avoid giving the solution before it is clear what the OP needs help with. these answers hold little to no value to future users, and we're not here to do the homework of others.

@Federico OK, will do next time! I thought it might be helpful to see how to compute fuel based on distance, ground speed and fuel flow in general...

In other words, your homework...

@Abdull, Federico was in no way attacking anybody. The community on this website tries to focus on explaining how to solve this kind of problems, rather than just providing the answer.

Sure, it can be helpful, but addressing specific things/doubts/questions can be more helpful.

@JuanJimenez if you say so...

Yes, @Abdull, both I and Federico say so, and I vote to delete this question.


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Are there any full-size airplanes flying with a Selig S1223 airfoil?
Are there any full-size airplanes flying with a Selig S1223 airfoil?

Not that I can see. Lots of talk about its suitability for heavy lift cargo aircraft but no actual flying examples.

Based on a quick perusal of the "Incomplete Guide to Airfoil Usage", my initial answer is no.  If you look up any of the big name manufacturers that are present on that list (Schempp-Hirth, Schleicher, Grob, Schweizer were my picks offhand), they use a mixture of Wortmann FX, NACA, and Eppler airfoils.  That said, the truer rationale comes a look at a Cl vs Cd curve, or how the lift and drag of an airfoil change with respect to one another.
I have a representative Cl vs Cd curve below (Cl on the vertical axis and Cd on the horizontal axis), where the purple line is a Selig S1223 airfoil, the yellow is a Wortmann FX 61-163 (used on a number of Schleicher sailplanes), and the green is the Eppler 603 (used on a number of Grob sailplanes).  Obviously, the S1223 produces far more lift than any of the other airfoils, and hits its most efficient point (maximum lift for minimum drag) at a very high lift coefficient -- just a shade over 1!

However, this is not necessarily a good thing.  If we work some numbers, it starts to become apparent.  Using a the definition of lift coefficient, we can estimate the typical lift coefficient () of a Grob 103 in trimmed flight something like this, where  is the density of air,  is the aircraft's wing area, and  is it's flight speed.

For a Grob 103, let's assume that it's flying at maximum gross weight (so maximize the necessary lift), so 5978 N, at sea level the density of air is 1.225 kg/m^3, wing area is 17.9 m^2, and its best L/D speed is 52.5 knots (27 m/s). Plugging all those in, we get a wing lift coefficient of 0.75.  Admittedly, this does not translate exactly into an airfoil lift coefficient, but it suits the need for a comparison.
Looking on the Cl vs Cd chart, this value is below the S1223's minimum drag point, and is actually where the S1223's drag is starting to spike.  However, it does hit right in the lower drag region of the Eppler and the FX quite well.  Furthermore, gliders aren't flying straight and level, which is what this calculation implies, but are, actually, descending (when outside a thermal/ridge lift/etc.), so the actual lift coefficient we need is probably somewhat lower, leading to even higher drag production from the S1223 but falling right into the "sweet spot" of the Eppler and FX airfoils' lift/drag characteristics.
In short, while the S1223 does have amazing lift characteristics (at lower Reynolds numbers -- these trends may not hold to a Reynolds number of 3,000,000, which would be closer to an operating sailplane), it is actually not the right airfoil for a regular sailplane.  For model-scale aircraft, however, it does have application, especially when wingspan is limited.

Thanks for all the info!  Looking at a tandem biplane/triplane with a span of 7' and

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What is this aircraft on the grass in Budapest International Airport?

While taxiing after landing in BUD recently, I noticed a widebody aircraft half-hidden in the trees on the grass of the airport. You can see it on Google Maps over here.
What is this aircraft, and why is it positioned like that? There’s the fire brigade building nearby, but they already have a training aircraft a lot nearer.
More 3D views:

I added some 3D views to help with identification. Someone should be able to easily find out from them once my edit is peer reviewed.

Is there some usual reason why its vertical stabilizer would be missing?

@Roger as per the first photo in DeepSpace’s answer, the vertical stabilizer is missing because it’s on display in the nearby museum

It has the distinct shape of the 767, and it is indeed a 767 (registration HA-LHB, ex Malev) as can be seen in these photos:

https://www.airliners.net/photo/Malev-Hungarian-Airlines/Boeing-767-27G-ER/4847571
https://www.airliners.net/photo/Untitled-Malev-Hungarian-Airlines/Boeing-767-27G-ER/4850697
https://www.airliners.net/photo/Untitled-Malev-Hungarian-Airlines/Boeing-767-27G-ER/4808155

By the state of the airframe (and the fact that it is illuminated) it is indeed looks like it is being used as a firefighting/rescue training frame.

It’s interesting that they would put it so far from the fire brigade building though. You have to go through or around  the patch of trees to get to the aircraft — there doesn’t seem to be a pathway to it.

@mcm69 - that might well be the purpose - in real emergency situations there won't be a nice easy path to the site, so one should train for it not to be easy.

At one of the airports I frequent (I forgot which one, it might be Frankfurt, Oslo, Bergen, or Lisbon), the firefighting training mockup sits right next to one end of one of the runways. Which means that during training, the last thing a nervous passenger is going to be see before leaving safe ground, is an airplane going up in a giant fireball right next to them. Might be another reason to put it out of sight.

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What do aircraft designers like Burt Rutan do to cheaply practice geting good at designing aircraft?
I am a new aircraft designer, and work for a small company (which doesn't have the enormous resources of a Boeing or Lockheed) so we have to work with limited resources to get good at our trade. I've been looking over different approaches for becoming a master designer, and for learning the shortcuts and tips all great designers pickup over the years of practicing their art.
From reading, it seems most good designers got their experience by essentially apprenticing with great engineers, or having the freedom to try stuff and see if it worked or not. I was thinking how a designer could get good in a small company like mine and came up with the following options:
1) Leave the company for a larger, better funded company like Boeing and come back
Pros: Have freedom to try stuff since company has the budget to absorb mistakes, and get to interact and learn from experienced engineers at company.
Cons: From all the people I talk to, nobody can take an aircraft (even a model) to full completion since the big companies compartmentalize the skills they pick up, meaning they good at making their widget or subsystem, but never work on integrating everything together to make the whole system.
2) Build a lot of R/C models to learn the fundamentals by flying, crashing, and repairing the aircraft
Pros: Get a ton of flight experience and building experience so will quickly learn what works, what doesn't work, and learn how to take aircraft from design to flight fully.
Cons: Aerodynamics do not fully scale unless the R/C models are somewhat large (around the 10-15 kg mass), which means crashes, even minor ones, cause a LOT of damage, which gets costly quickly. Replacing busted motors, shafts, wings, and the time that is needed to do this can quickly eat into any real experience.
3) Use a digital flight simulator like X-Plane or Kerbal Space Program to learn what flies and doesn't
Pros: This is the ultimate for cheapness. I can design and crash an infinite amount of times at no additional cost, as well as fly the aircraft at full scale and see all of the response characteristics in a variety of conditions.
Cons: Building the aircraft in the physical world does not mirror how its built in the software, meaning a lot of new factors (like weight gain, manufacturing inaccuracies) come into change how the actual aircraft flies compared to the simulator aircraft. Also, the simulator has limitations in its physics model, so things like stall behavior or transsonic drag will differ significantly between simulator and real life.
4) Use a hybrid system, using 3D printing, modular parts, R/C aircraft, and flight simulators
Pros: This will teach me how to take a design to completion by first designing/flying aircraft in simulator using modular parts (like in Kerbal Space Program), then print those modular parts (wing sections, fuselage sections, wing/fuselage joints, etc.) in real life using 3D printers, and assemble the aircraft at small R/C scale to fly, before finally going to full scale using the same modular parts, only scaled up. It lets me make mistakes and learn without bankrupting the project since I learn how to avoid the big mistakes while the consequences are still small.
Cons: I will need to design and build modular blocks light enough with the same performance as that in the simulator at both R/C and full scale. Also, while somewhat easy for airframe components, building engines this way would still be costly, as some combustion engines have different performance at small scales than large scales (heat leakage and manufacturing tolerances become more important at the small scale than at large scale) meaning it might cost too much to build the system of modular parts.
What option did designers like Burt Rutan use for getting good at aircraft design?

Where should I repost it?

ok I'll do that

Where are the opinion areas I need to delete?

The title and last sentence are a valid question, of how Burt Rutan (or others) gained experience. The list of 4 options seems to be more asking for career advice, which will at best be someone's opinion.

So are you more of civil aircraft designer or military? I also favorite this question.

I am more of a civil aircraft designer.

Let's look at every of your concepts:

Joining a large company: As you noticed yourself, you won't get a broad range of skills, but a deep understanding of a narrow field, unless you rotate between departments every few years. This will be hard to pull off, and will take time. If you are good at what you do, your boss will try to keep you (they always try to promote the duds away in big companies, and the good people get stuck. Really!)
Building RC models is great if you want to become a configuration expert at low speed aerodynamics. You will gain no experience in transsonic aerodynamics, structure, systems - 90% of the needed knowledge cannot be gained from model aircraft. But after 20 models you will be great at understanding flight mechanics and the pros and cons of different configurations.
Forget flight simulators. Things only get interesting once you leave the realm of attached flow. Stalls, spins, flutter, oscillating shocks - that is what you need to understand when you want to do good work, and flight simulators will not teach you this. At all. Also important is to understand the effect of single failures on the whole aircraft, and designing for robust response. Again, no simulator will give you anything even resembling a realistic response.
Hybrid: Now we are talking: Keep your eyes and ears open! Visit conferences, museums, exhibitions and talk to the people. Get a pilot's license and fly yourself! Read books and technical reports. Be open to all things, be it metallurgy, fuel chemistry or whatever. Try to do stints in workshops and flying clubs. Volunteer for workshop duty and do maintenance and repairs. Don't be shy to ask, and if you see something strange, don't stop until you have arrived at a convincing explanation. Do this 10 hours a day for 10 years, and you will collect the necessary all-around knowledge.

The best way will be to participate in as many aircraft design projects as possible, from start to finish. This has become extremely hard, and some engineers will spend their whole professional life working on just one design. Let's face it, the golden days where you would work on a new design every year were maybe 80 years ago. On the other hand, the Internet can bring you an incredible wealth of information and past experiences from which to learn. Like this site. Use it!

What advice can you give me at my age to become an aircraft designer.

@Ethan: Go to university and get a good degree in aero/astro. Until then, do as I recommended to user11377.

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Why is the F-104 Starfighter’s afterburner thrust not greatly impacted by altitude?
Here is a page from the F-104 flight manual. It shows the flight envelope for the F-104 Starfighter with the powerful J79-GE-19 turbojet engine:

I have a question regarding the shape of the afterburner envelope.
In Aerodynamics for Naval Aviators, published in 1965, the author, Hugh Harrison Hurt, Jr., states:

The thrust added by the afterburner of a turbojet engine is not
affected so greatly by altitude as the basic engine thrust. The use of
afterburner may provide a thrust increase of 50 percent at low
altitude or as much as 100 percent at high altitude.

Why is military thrust (basic engine thrust) greatly impacted by altitude, and afterburner thrust is not?

The airplane needs more thrust to fly higher, and/or faster, and of course the AB provides more thrust. So the envelope of attainable speeds & altitudes with AB thrust is larger than what's attainable with mil power. But that's as much about the wing as it is the engine. What in that graph leads you to believe that it shows the thrust of the engine in mil power to be  more affected by altitude than the thrust of the engine in AB power? Or is the question really about the Hurt quote itself? This graph may not reveal much to explain the reasoning behind Hurt's statement.

Wow I didn't know the 104 could supercruise on military power.

His question clearly relates to the quote, and the chart shows something interesting insofar as the ceiling actually increases with the mach # increase.  That part is all in the stratosphere so the temperature isn't really decreasing between 45 and 65000 ft.

@JohnK Nobody would sensibly cruise at Mach 1.02. Either stay at 0.9 and save loads of fuel or go all the way to 1.4 or higher.

What I meant is I was surprised it could exceed mach at all on mil power.  Also it was without stores so in practical terms it's a moot point.


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Why do some A380 main wheels have no brakes?
The A380 has 20 main and 2 nose wheels. Apparently the nose wheels have no brakes (compare with Are there any aircraft with a nose wheel braking system?).
While researching for this question (How are brakes cooled on heavy aircraft?), I assumed that all of the 20 main wheels are braked. I looked at a few pictures of A380 landing gears. As far as I have seen, all main wheels are equipped with brake cooling fans (BCF), except the four rear wheels in the inner 6-wheeled  undercarriages. (e.g. here).
This picture below clearly shows that there is no brake unit inside the rim:

There is another picture here whithout BCFs, but the same four wheels apparently have no brakes.
What is the reason for omitting the brakes at these four wheel positions?
Is this a common configuration, or quite unique to the A380?

I've got no facts for this, so a comment:
Here you can read that carbon brakes have less wear when used to apply heavy braking forces. Perhaps some of the brakes are omitted, such that the others have to 'work harder' and thus, wear less?

Because it doesn't need them.  Why fit what's not needed?

According to this presentation by Claude Lelaie (Head of A380 test), the brakes of the A380 were tested before certification with 89 MJ, and after certification with 120 MJ.
According to your calculation here the RTO of an A380 sinks 1.7 GJ to the brakes.
Therefore either configuration would be sufficient with respect to the RTO energy:

20x  89 MJ = 1.78 GJ
16x 120 MJ = 1.92 GJ

It is not clear if this setup was planned from the beginning, or maybe the brake design turned out to be better than expected.
During braking, the vector sum of brake and weight forces is pointing forward. This means increase of force on the front wheels, and less force on the rear wheels. This is probably the reason why the engineers have chosen to omit the brakes on the 4 rearmost wheels, and not a different location.

Because they are not needed (for the passenger variant). Addition of brakes increases the weight and complexity of the system, which is undesirable. Why add something that is not required?. If it is required, additional brakes can always be added, like in the (proposed) freighter version. According to flightglobal.com:

Sixteen of the A380-800's 20 main wheels are braked, with the two aft wheels on each body landing gear of the passenger version not having brakes (unlike the freighter variant, where all 20 main wheels are braked due to its increased weights).

Also, Aft Axle of BLG is steerable to ensure better maneuverability for small radius turn (Body Wheel Steering); braking a steerable axle would require special linkages, increasing weight and complexity. This is not insurmountable, however- Boeing 777's landing gear is similar, but has brakes on all its wheels.

@bogl The 777 only has 12 main gear wheels (6 on each). The brakes on the A380 aren't needed because the aircraft's landing weight doesn't require that much braking force for the runways that it needs to land on. The freighter version would be different since presumably it would land at higher weights. Similarly the A380 doesn't have thrust reversers on the outboard engines either.

Out of curiosity: would the 'needed' braking force account for action of thrust reversers? (I'd imagine so under normal operation, but that the braking system could do the job in the event of failure at the cost of higher wear?)

@msanford The aircraft is required to be able to stop without the use of thrust reversers, if that is what you are asking.

@RonBeyer I assume this to be the case. I guess that does make my question rather hair-splitting, since it's obvious that thrust reversal would reduce brake wear at least somewhat. Thanks!

@bogl The aircraft must be able to come to a complete stop from V1 on the remaining runway without the use of thrust reversers.

@bogl Your V1 speed will depend on the runway lenght. It is not a fixed value. So yes, any runway.

@J.Hougaard The shorter the runway, the lower V1, the easier for the plane to pass the RTO brake test? That cannot be.

@bogl The aircraft also needs to be able to take off on that runway because the tests are run at MTOW. There is no point in using a runway shorter than the certified MTOW requires.

@bogl The actual RTO certification test isnt done based on runway length, its done based on the maximum kinetic energy dissipation of the braking system the aircraft manufacturer wants the aircraft certified to.  Page 33 on https://www.faa.gov/documentLibrary/media/Advisory_Circular/25-7B.pdf

@Moo Great! This is what I was looking for!

@J.Hougaard No, V1 does not depend on the runway length.

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What is the effect of flow separation on lift, pressure distribution and drag?
Consider an airfoil (a wing) in an airflow. When angle of attack increases, at some point the flow start to separate from trailing edge. By increasing the angle of attack more, this separation area increase toward the leading edge up to stall condition which must be avoided. We know that it will cause to reduce the lift and increase the drag suddenly.
But i don't remember what was the exact reason behind this. Whats the magnitude of pressure inside separated regions of flow (wakes)? is it higher or lower than normal flow patterns before separation occur?
I read (for example here) that separation increase 'Pressure Drag' as the pressure inside wakes are lower(?) and we get a more net back force comparing front side and back side of the airfoil:

In aerodynamics, flow separation can often result in increased drag, particularly pressure drag which is caused by the pressure differential between the front and rear surfaces of the object as it travels through the fluid

But if this is true, then how could we say that this lower pressure in upside of the airfoil decrease the Lift? as the lift is produced from pressure difference of top and bottom of the airfoil.
Note: i can explain the reason using newton third law and change in flow directions. but not sure about exact pressure profiles.

Based on my understandings, the tip is about this fact that most of the lift is produced by initial sections of leading edge (eg. the first 20% of the chord) as shown in this figure.
Now compare the green(before separation) and pink (after separation) plots in the following figure.
Before separation, the pressure near leading edge (first 20% of chord) is very lower than ambient pressure (eg. -5 to -2), but after it the pressure vary linearly toward the ambient pressure such that at the trailing edge it is about 0.
After separation, the pressure all over the wakes change toward a (constant) value which is a little lower than the ambient pressure (e.g. -1 as shown for pink plot) as the wakes are some circular flow independent of main stream which pass above them.
This means an higher pressure in the first 20-30% of the chord, but lower pressure on the remaining 70-80% of the chord (when compare green with pink lines).
So, the suction of the wing at its leading edge region (20-30% c) reduces which means less lift (as the most portion of lift is produced by this section).
But the remaining 70-80% of the chord will have lower pressure which increase the net pressure differential between the front and rear surfaces of the airfoil and so increase the pressure drag.
Note also that this 70-80% of chord is placed at a high angle (angle of attack) and so has a great effect when we compute its horizontal component (drag). In the other side, that first section of airfoil leading edge is approximately an horizontal surface so the pressure vector is most normal to airflow and has more effect on lift when we compute its vertical component.

You need to distinguish between boundary layer flow and outer flow.
Separation means that a parcel of low energy air moves with the wing (here the pressure is about -0.2, expressed as a pressure coefficient). For the outer flow, the contour of the airfoil changes to one of the original airfoil plus the parcel of separated flow. Since this is on the rear upper side of the airfoil, the airfoil "looks" thicker and longer, effectively presenting to the outer flow a body of less camber and angle of attack.
This in turn will reduce the suction in the region of attached flow right past the nose, where normally a high suction peak produces both lift and nose thrust. With a reduced suction peak, the airfoil produces more drag and less lift.

Viscous and inviscid pressure distribution at 12° AoA, calculated with XFOIL 5.4. The dashed lines show the inviscid pressure distribution where flow separation does not happen, while the solid lines show the viscous pressure distribution with separated flow over the last 20% of the upper side. The lines around the airfoil contour show the boundary between the boundary layer and the outer flow. Separation thickens the boundary layer, substantially altering the shape around which the outer flow streams.
The altered airfoil shape causes a reduction in the effective angle of attack, so both the suction on top and the pressure on the bottom are lower. This means that separation causes lift loss; once separation starts, the lift curve slope becomes flatter and can reverse (which indicates a full stall).
In the sample shown above, only on the last 12% of the upper side would separation cause pressure to be lower than in the inviscid case. Note that pressure on the bottom is also lower, so the difference between both (which causes lift) is quite unchanged by separation. Without separation, viscous and inviscid pressure distribution are quite similar, therefore the inviscid distribution is a valid first-order approximation of the viscous pressure distribution without separation.
The reduced suction peak at the nose and the higher suction on those last 12% is what causes more pressure drag. Note that the area around the suction peak points forward and that near the trailing edge points a little backward (especially at high angle of attack). This means that less of the nose thrust pulls the airfoil forward and also there is less pushing from the pressure near the trailing edge.
To estimate the drag increase due to separation of the full airplane configuration, assume a drag increase as if the lift would increase linearly, without separation taking place, including induced drag. Combine this drag with the "real" lift, including losses due to separation, and you will arrive at a surprisingly realistic lift-to-drag ratio of the aircraft between separation onset and stall.

thanks for your explanations. but this is not what i asked exactly. this does not answer the paradox of: reducing pressure above the wing produce more pressure drag because of left/right pressure difference, but how could it reduce the lift (we expect less pressure on upper surface produce more lift)

@S.Serp: I guess I need to add pictures. What do you mean by left/right pressure?

i mean the front and rear surfaces of the airfoil as it travels through the fluid (as usually flow considered in horizontal direction, it become left and right sides)

can you also take a look at my answer and improve your answer or provide your comments? i appreciate your effort and knowledge as i see many technical questions are answered by you in this site! +1 for your third paragraph as it make sense to me now, specially the suction peak and nose thrust phrases (better if add some pics as you said). but the last paragraph is not so clear.

@S.Serp: The last paragraph is for drag estimation of the full aircraft once separation starts. It does not explain the changes in lift (which is what the rest of the answer tries to do) but the increase in drag.

@PeterKämpf Looking at the outer flow I understand why the pressure in the above distribution is constant after the separation point. However, I'm wondering how this is transferred to the surface of the airfoil. In a boundary layer, we typically assume that the static pressure doesn't change perpendicular to the surface. Does the same thing hold for a separated boundary layer with a large region of reverse flow?

@Daniel: As far as I know, yes. The speed of the reverse flow is slow, so there isn't much change in pressure possible.

From "Fundamentals of Aerodynamics" by J.D.Anderson Jr (Fifth edition, page 384, 385). I just copy and paste in the case you don't have the book:

Here the airfoil at a large angle of attack (thus with flow separation) is
shown with the real surface pressure distribution symbolized by the solid arrows.
Pressure always acts normal to the surface. Hence the arrows are all locally
perpendicular to the surface. The length of the arrows is representative of the
magnitude of the pressure. A solid curve is drawn through the base of the arrows to form an “envelope” to make the pressure distribution easier to visualize. However,if the flow were not separated, that is, if the flow were attached, then the pressure distribution would be that shown by the dashed arrows (and the dashed envelope).
The solid and dashed arrows in Figure 4.47 should be compared carefully.
They explain the two major consequences of separated flow over the airfoil. The
first consequence is a loss of lift. The aerodynamic lift (the vertical force shown
in Figure 4.47) is derived from the net component of the pressure distribution in
the vertical direction in Figure 4.47 (assuming that the free stream relative wind
is horizontal in this figure). High lift is obtained when the pressure on the bottom
surface is large and the pressure on the top surface is small. Separation does not
affect the bottom surface pressure distribution. However, comparing the solid and
dashed arrows on the top surfacejust downstream of the leading edge,we find the
solid arrows indicating a higher pressure when the flow is separated. This higher
pressure is pushing down, hence reducing the lift. This reduction in lift is also
compounded by the geometric effect that the position of the top surface of the
airfoil near the leading edge is approximately horizontal in Figure 4.47. When
the flow is separated, causing a higher pressure on this part of the airfoil surface,
the direction in which the pressure is acting is closely aligned to the vertical,
and hence, almost the full effect of the increased pressure is felt by the lift. The
combined effect of the increased pressure on the top surface near the leading edge,
and the fact that this portion of the surface is approximately horizontal, leads to
the rather dramatic loss of lift when the flow separates. Note in Figure 4.47 that
the lift for separated flow (the solid vertical vector) is smaller than the lift that
would exist if the flow were attached (the dashed vertical vector).
Now let us concentrate on that portion of the top surfacenear the trailing
edge. On this portion of the airfoil surface, the pressure for the separated flow
is now smaller than the pressure that would exist if the flow were attached.
Moreover, the top surface near the trailing edge geometrically is inclined more to
the horizontal, and, in fact, somewhat faces in the horizontal direction. Recall that
the drag is in the horizontal direction in Figure 4.47. Because of the inclination
of the top surface near the trailing edge, the pressure exerted on this portion of
the surface has a strong component in the horizontal direction. This component
acts towards the left, tending to counter the horizontal component of force due
to the high pressure acting on the nose of the airfoil pushing toward the right.
The net pressure drag on the airfoil is the difference between the force exerted
on the front pushing toward the right and the force exerted on the back pushing
toward the left. When the flow is separated, the pressure on the back is lower than
it would be if the flow were attached. Hence, for the separated flow, there isless
force on the back pushing toward the left, and thenetdrag acting toward the right
is therefore increased. Note in Figure 4.47 that the drag for separated flow (the
solid horizontal vector) is larger than the drag that would exist if the flow were
attached (the dashed horizontal vector).
Therefore, two major consequences of the flow separating over an airfoil are:

A drastic loss of lift (stalling).
A major increase in drag, caused by pressure drag due to flow separation.

thanks for your answer. but from the pressure profile of your figure  its not obvious why drag increases. based on your figure, it seems that the pressure increase in most portion of airfoil which will reduce the drag. based on drawn geometry, we can't say easily only last portion of trailing edge is affecting drag, the curvature slope is almost the same for most part of airfoil after its initial portion

You are right, I have read this part of the  book but still not fully understood the author's idea, it's kind of vague. Need a better answer...


text:
Do older aircraft require increased general maintenance?

Does the age of the aircraft increase the aircraft's maintenance expenses, fuel consumption, and electrical shorts?
If so, by how much?
And is this a reason why planes get scrapped?

Could you try to ask a specific, clear question, please? It's hard to give a useful answer without understanding exactly what you want to know.

Simple answer: it's like a car. More maintenance, and less modern components. Old design means lack of recent progress in efficiency. They can fly, but cost more and do less. If you focus your question on a single specific aspect, then you may get accurate answers.

The very simple answer to your first question is just yes.
As something (or someone) age, more maintenance is required. It varies from case to case and you cannot have one size fits all rule or guideline.
FAA has published a thorough guide for maintaining older airplanes (PDF). For the relation of age on maintenance costs, this article (PDF) discusses it in detail.
Getting to your last question, it also depends on an individual case. Please also note that when an airplane is grounded, it is not required to be scrapped in all cases. Sometimes museums are interested in it, or it can be installed as a roadside monument or placed in an aircraft boneyard.
Regarding when an aircraft needs to be retired or scraped, these two articles are a good read:

When to replace an aircraft
How old is too old?

It can also be a runaway-side monument: https://commons.wikimedia.org/wiki/File:Concorde_expo_CDG.JPG

Farhan poses some great points and offers some great articles but for the sake of the other side of the answer I will offer up some other points mainly related to GA.
While the general answer is yes, I will offer an answer that "Hours are more important than years".
From a pure engineering standpoint wear and tear is more a function of cycles than time. An new aircraft that is used as say a trainer and sees lots of hours a year may be in worse shape than a privately owned rarely flown aircraft from 30 years ago. To you points,

fuel consumption (and oil consumption for that matter)

Both more a function of the engine than the airframe. Engines are overhauled in and around 2000 hours for GA planes (or if there is an incident or problem). A freshly overhauled engine will (or at least should) preform like a brand new engine and should burn oil and fuel as such. Depending on how often you fly there are plenty of very old planes out there with low hour overhauled engines that run like a top.

maintenance expenses

In general yes older planes will cost more to keep up as they may see more broken parts due to use. On the other hand older planes tend to be simpler and parts are often cheaper. One could argue its cheaper to upkeep an older Piper Arrow than a brand new Cirrus (granted these are very different planes). But the Cirrus sees a very pricy annual as a result of its ballistic chute and I'm sure the carbon fiber is not cheap to fix if something goes wrong.
There is another point to note and this very much depends on the plane but lets break GA aircraft into 2 categories. All metal (or composite) and fabric/wood planes. It should be obvious that fabric covered planes are not as durable as their metal counterparts and can be harder to leave outside in weather which means you will need a hangar. Wood planes or planes with wood components (early Mooney's) suffer from rot and cracked wood. Then again metal planes can suffer from rust and corrosion. You may also be hard pressed to find a mechanic who knows the ins and outs of a wood plane these days.
The sweet spot seems to be the early but not first all metal planes (think Piper Cherokee). These planes are simple and easy to fix, contain parts that are still made in many cases making them cheap to own. You can buy a beat up 140 for the price of a composite 3 blade prop for some of these new planes. Granted you won't go nearly as fast, you will still fly and you will still own a plane.

Short answer; yes.
Longer answer:
As with any complex mechanical system, there are several overlapping service lives of various subsystems. Simply put, the longer a machine is in operation, the more wear and tear is put on parts of the machine that aren't designed to wear out and be replaced as part of normal maintenance (like brake pads, belts, hoses, etc). You replace your car's tires every 30k-60k miles, but the drivers' side door isn't something you just plan to replace after X miles. Nevertheless, replacing or refurbishing the driver's side door after 20 years will probably give you a noticeable improvement with regards to all the things that door contains, from the hinges it opens on to the gasket that seals it against rain and noise, to all the switches, motors and servos controlling the window, lock, mirror, etc.
The same is true for an airplane; the longer the airframe is in service, the more things that aren't designed to wear out do so anyway. The difference is that with a car, if a wheel falls off or the radiator hose blows out, you just limp to the side of the road and call a tow truck. Failures on an aircraft are not so easily handled, and remember the mantra; taking off is optional, landing is mandatory. By the same token, while car maintenance is frequently stretched far more than is prudent, because the gamble is against an inconvenience of a few hours and the tow truck fee, maintenance on an aircraft is a no-nonsense business because the gamble of not doing it is the lives of a hundred-plus people aboard when something goes wrong; inspections with accompanying repairs, replacements and retrofits happen at set intervals of flight hours or press/depress cycles, like clockwork, without fail, or the plane doesn't fly.
That means the longer the airframe is in service, the more things start to reach the point where they must be replaced, either because they've worn out or are obsolete. Eventually, between the cost of replacement and the lost revenue of the plane being out of service to fix the ever-increasing number of failing components, it's no longer good business to continue operating that particular airframe, at which point it's sold, either in flyable condition to a discount airline, or for parts/scrap at a boneyard.
In addition, newer planes have a lot of advantages simply because they were developed using better technology. Engines, for instance, are a component of a plane that is not easily upgraded to an entirely new design, because of considerations like overall aerodynamics and under-wing clearance. While some upgrades are possible (the 737-100 had a much smaller, less efficient engine than modern 700s and 800s), they're expensive to apply especially to a fleet of hundreds of planes, and they often involve tradeoffs like reducing cargo area or increasing pilot workload due to less integration capability of the new system with existing avionics. As a result, turnover of an airline's fleet is a virtual necessity to continually improve the fleet's performance as well as avoid end-of-life mechanical problems.

Excellent answer, but I would add that as an aircraft ages, many times the components are no longer even manufactured, so prices go higher and higher on refurbished and repaired units as they become more and more scarce.  This is a large source of the increased cost of maintaining an old aircraft as well.

Your answer -and the others to this question- are making me think the circumstances of commercial aviation on one hand, and GA on the other are quite different. Which is coherent with the fact that one sees very few 40-year-old passenger jets in daily use, but a rather larger proportion of GA aircraft of the same age.

@ALANWARD - That's very correct. Most light singles are designed with a rugged simplicity, which lowers maintenance costs. The biggest savings is that the plane is unpressurized, meaning the airframe just needs to be rated to carry the weight of passengers, cargo and required G-loading, instead of additionally needing to handle the pressure differential an airliner does. The average number of flight hours per day is also fairly low; many of the maintenance issues you see on older GA airframes isn't from overuse, but weathering and neglect, where airliners are flown 16 hours/day every day.


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Could a runway include a safety device to destroy a plane's landing gear?
The NTSB recently released its final report of a crash at Hanscom field (KBED) in Bedford, MA. The short of it is that the pilot tried to rotate with the gust lock engaged, preventing the plane from lifting off. It rolled past the runway and finally crashed into a ditch, where it burst into flame, killing everyone on board. (I should say that although I've flown in a small 4-seater a few times, I'm not a pilot — so please forgive me if I misused some terminology.)
One thought I had is that if the landing gear had been destroyed at the end of the runway, the crash could have been a lot less severe. In particular, I was thinking that a small wall could have been designed to knock the gear out, making the plane start skidding on its belly and presumably coming to a stop much sooner. It wouldn't be exactly safe, but it seems like it would be safer than relying on just the gear's braking power.
Does such a device exist at any airports? And if not, has it been considered and rejected for some reason?

hitting the brakes gives more stopping power than sliding on the belly

An arresting material is very sensitive to the weight of the airplane:  A surface that would give way under a 767 would essentially be just an extension of the runway for a Cessna Citation; and material that would stop a small-medium private plane would offer minimal resistance to a jumbo jet.  So they are not appropriate for airports with diverse types of traffic.

@abelenky What about a multi-layered material? The first layer would be thin and designed to arrest the Citation; the next layer would be a bit more firm and designed to arrest the small-medium plane; and the layer below would be designed to arrest large planes. A large plane would easily tear through the first two layers before being arrested by the third. I imagine the biggest inhibitor there would be cost?

A related concept is the Runaway Truck Lanes that are placed at the bottom of large hills on highways that can be used to stop a truck or car whose brakes have failed.  Like the EMAS of an airport, it allows the truck tires to sink into a material that slows the truck without catastrophic damage.

Just a note... I think the word "gears" is inappropriate here, as the plural of "gear" (in the sense it is used here) is "gear".  My first thought on seeing the title on the hot list was, "What? Planes don't have gears!"

The landing gear assembly is the strongest part of the plane. You can't "knock it out".

In their briefing, the NTSB noted that lights and antennas on the ground "likely substantially damaged" the aircraft in question - and that it's FAA policy to use frangible (breakable) fittings, so that objects on the ground don't damage aircraft which overrun a runway.

There is indeed such a system installed at some airports. It is called an engineered materials arrestor system (EMAS). Incidents such as Southwest 1248 have shown the potential safety issues of runway overruns, as you have pointed out. The EMAS solution uses a pad of special material at the end of the runway, designed to allow landing gear to sink into the material. The landing gear is not ripped off, but as the gear travels through the material, the plane is slowed down.
Destroying the gear is not a desirable option. Although the gear on airliners is designed to safely separate from the aircraft, there is still the potential for damaging fuel tanks or other structure. The aircraft will also be even less controllable after the loss of landing gear. If a small aircraft is not going very fast, a small wall could cause it to flip over, worsening the situation.
In the crash you referenced, the aircraft was traveling very fast at the end of the runway, and an EMAS may not have been sufficient to stop the plane. An EMAS typically has a max entry speed of 70 knots, so in incidents like the one linked from the question, this kind of system may not completely prevent a fatal incident. Since the aircraft's gear must break through the material, lighter aircraft may not be stopped as effectively. However, small aircraft have been stopped with such a system.

The FAA is looking at places where these systems could be useful, with a focus on runways that cannot have the recommended 1,000 foot safety area at the end. The runway safety area (RSA) is the best option for a safety buffer at the end of a runway, providing extra distance for an aircraft to stop. The article above notes that the EMAS at Burbank airport took $4 million to install, and that is a fairly small airport that doesn't usually see anything larger than a 737. Any overrun will incur certain recovery costs, but an EMAS will need to be repaired after any overrun or accidental damage, possibly closing the runway, and will be less effective until repairs are completed. Airports must weigh all of their options for providing a safe facility.

Also: http://gizmodo.com/5869715/why-did-this-airplane-landing-gear-destroy-this-concrete-runway

Small followup: is there ever a reason not to include such a system, other than cost?

@yshavit: See my note above, about various weights/types of planes.  Also, at an airport that may have a lot of students, minor overruns happen. If the runway just has fields beyond the threshold, it'd be preferable to drag a plane out of the mud periodically rather than frequently repair your arrestor system.

This. EMAS systems are very expensive to have to replace tiles every time some tyro in a 150 or Cherokee lands a bit too deep. Similar ideas, like sugar sand pits, are more reusable (just rake them back smooth) and more effective for this kind of runoff, but less effective than EMAS would be for a runoff under power.

In addition to the costs incurred after an overrun (inspection and replacement of damaged tiles) EMAS beds require routine periodic inspections to ensure the tiles have not been damaged by weather or other factors, and incur their own maintenance overhead (above what would be required for a simple blast pad / stopway and "adequate" clearway beyond.

In that picture it almost looks like the plane tried to enter a section where the concrete hadn't cured yet... ;-)

The short answer is yes; EMAS and other arresting systems up to and including nets are in place at many airfields of varying sizes. The most common AFAIK is a gravel pit, aka the litterbox; it's filled with sugar sand, a lightweight gravel like pumice rock or a manufactured alternative, that yields and/or crushes much like EMAS panels under the weight of landing gear to increase rolling resistance and stop a plane that rolls off the end of the runway. They are not designed to damage the plane such as by snapping the gear off (though such damage is often par for the course), as that would actually take away the plane's most effective stopping method, its brakes and rubber tires, instead forcing the plane to skid to a stop on its slick underbelly. They're also definitely not designed or intended to stop a plane barreling off the end of the runway under full engine power.
There is no safety mechanism in existence that can 100% replace the human brain in ensuring safe performance of inherently dangerous tasks. In the accident you refer to, several basic yet critical checklist steps were skipped that would have indicated the presence of a gust lock, among them visually inspecting and checking for free movement of control surfaces during the walkaround, and the stick/rudder movement check during runup to ensure the flight controls are unhindered and properly manipulate the control surfaces. The NTSB places the blame for this accident squarely on the failure to perform basic preflight checks prior to the attempted takeoff. Preflight checks exist for this exact reason; to prevent pilots attempting to fly a plane they will not be able to operate safely due to some mechanical defect or hindrance, whether that's a gust lock, water in the fuel tanks, a bird nesting in the engine cowling, or a bullet hole in the rudder. The walkaround is literally the first thing a new civilian pilot learns on their first flight.
There really is no such thing as a foolproof system; fools are too ingenious. The pilot(s) of an aircaft are ultimately responsible for the safe operation of that aircraft, because it's ultimately their lives depending on a safe flight. That's why it's part of their job to do the preflight checks. If a pilot doesn't value his own life enough to do something as basic as this, he shouldn't be anywhere near an aircraft, because when his lack of regard leads to the loss of his life, it's very unlikely his life will be the only one lost.

fools are too ingenious. +1 for this alone!

Yeah, technically fools are not too ingenious, they are simply so numerous that you can't catch them all, but well said. Especially since that actually is the root issue here.

@VilleNiemi - You apparently haven't seen the lengths to which people will go to willfully ignore or circumvent safety systems. Douglas Adams said it best, in the Hitchhiker installment Mostly Harmless: "a common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools".

@KeithS Good point. It is astonishing to remember that Chernobyl required, despite its obsolete and exotic design, deliberate effort on part of its controllers before it would blow up... The safeties did not fail, they were deliberately turned off. I wonder,since fools are both ingenious and numerous, why don't they rule the world? Oh, wait...

"EMAS and other arresting systems up to and including nets" I think you mean "down to and including", :-)


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Can a software "hard freeze" or serious hardware problem completely disable the whole cockpit?
I think that the glass cockpit of a modern plane has some sort of a central power supply or at least some central controlling software.
What happens if this central power suddenly disappears, or the central software freezes? Yes, of course, this would be a severe failure and would put everybody's life on the plane in serious danger.
Is such a failure possible?

I see that you removed the speculation.  I have reversed my close vote.

Thanks for removing the speculation and please do not add further questions, or manipulate existing questions, to speculate on any real incident.  It is a very sensitive subject and no-one here wants to comment until the accident report is published.  We have no way of knowing if you, or any other user, are actually a journalist who may quote something said on this site.  I am not saying that you might be, just explaining why we don't like speculation here.

I remember to have heard about an Airbus whose Displays (not sure if it was the whole system) shut down during a maneuver where the sensors of the airplane sent values to the computer that it thought must have been wrong (gotta be one heck of a maneuver) and thus did a restart to reboot all the sensors. I think this Display failure last for about 3 seconds, then everything was back up and working. I'm still searching for a report though, so don't quote me on that! ;)

@Maverick283 That's not a bug, that's a feature. When sensor informations are thought to be inaccurate, aircraft try to prevent it to interfere with pilot action and decision process by... Removing it.
Some pilots use tape to hide non-glass cockpits instruments once they are believed to failed.

@Antzi Jupp totally agree didn't mean to make it sound like  a bug...

@Antzi -- suction cups are common as well (both for practicing partial panel ops and hiding a dead or lying instrument)

Is such a failure possible? Yes. Is it probable? No.

Apparently happened recently:  http://avherald.com/h?article=4aaf9a3a&opt=0

If the power suddenly disappears to any aircraft (ie, a total electrical failure including the batteries), then all the aircraft systems will be failed regardless of whether or not they were glass.  This is highly unlikely because there are several power generation backups that would have to fail, including the batteries themselves, before this ever became a reality.  Unfortunately though, if the aircraft only has electrically controlled, hydraulically actuated control surfaces (fly-by-wire), vice actual mechanical linkages (or backup controls), then a total electrical failure would be 100% fatal if power cannot be restored.  The control surfaces would not be movable.
However, more to the question regarding software bugs, all primary flight instruments are backed up to prevent such disasters from happening.  I have personally lost all the digital displays in my cockpit and flown a no-gyro, surveillance approach in bad weather using nothing but a peanut gyro.  Its possible, but definitely not a good time.
Anything is possible.  However, even though the odds of a total electrical failure, or complete loss of instrumentation (including backups) is nominally greater than 0%, the statistical probably can still probably be rounded to 0.

No aircraft where no control surfaces would be movable without electric power exist! In most large aircraft they are not movable without hydraulic power though.

@JanHudec The Super Hornet is one such aircraft.

Hm, true, aerodynamically unstable fighters would not be controllable with mechanical backup anyway. It does apply to airliners though. All airliners have (hydro-)mechanical backup.

@RhinoDriver (sorry for the late react) I think these various power sources are selected and regulated by some unit, which decides when, from where to transport the voltage into which direction. This unit/device is a critical part of the plane, if it dies, everything dies. It would be possible to make also this redundantly, for example to have at least 2 of them, but it would much more than double the complexity of the circuitry. And more part means also more fault...

Not all systems would fail with a total electrical failure.  The altimeter, airspeed indicator, magnetic compass, and turn coordinator (none of which require electrical power) would still be operative; on aircraft with air-driven gyros, so would the navball and directional gyro.

While total electrical failure in a transport-category aircraft is extremely remote, even most fly-by-wire airliners such as the A320/330/340 and B777/787 have redundant backups that would work under the scenario: in the case of the Airbus, you'd fly using the rudder pedals for yaw and roll and manual stabilizer trim for pitch, while Boeing uses a pair of spoiler panels for mechanical roll backup instead of the rudder.
All of this, of course, depends on the presence of actuator hydraulic power - if you have no hydraulic power and no electric power on a FBW Airbus or Boeing (including a loss of the dedicated FADEC/EEC backup permanent magnet generators), you're toast because not even "fly by thrust lever" a la UAL232 and OO-DLL will save you in that case.

If all hydraulic and electrical power fails at once, then you're probably firmly in "hypothetical scenario" land (and at some level, all planes will crash if enough stuff fails).

@cpast -- closest I've seen would be the Aloha zip-top 737 -- I suspect the controls on that plane did go into manual reversion...

A total electrical failure assumes you don't have PMGs or battery power, in which case you wouldn't have FADECs either. To end up in this scenario you'd need to have a bad battery during the same flight you shed both gens and then shredded both AMADs leaving you with no PMGs. To add insult to injury you'd also be flying via rat powered hydraulics, so life would suck a lot. Maybe if you got hit with a manpad... but even then you probably wouldn't have a wing to fly with anyway.

@RhinoDriver -- yeah, I suspect this might be worse than a mere MANPAD that hit you...more like a SA-2 with a bad prox fuse poking a hole in your lower forward fuselage then exploding!

I think that if both engines explode they would take out most of the power and hydraulics, and possibly spoiler control.

@raptortech97 -- a double uncontained engine failure would be luck of the draw, yes -- it depends on the plane whether you still might have a hyd or not though.  For instance, an A330 would automatically be in bad shape in that case (all the hyds on that plane are driven off one engine or another), but an A320 would have a chance (it depends on whether the blue system survived) and the same holds for the Boeing non-reversion twinjets as their center systems do not have an engine driven pump.

@UnrecognizedFallingObject:  The green hydraulic circuit can also be driven by an electric pump or by the RAT if both engines are dead. A330 has manual control of stabilizer and rudder when all hydraulics are gone. You'd be in bad shape but you'd still be able to fly for a while.

@RedGrittyBrick -- an uncontained failure can mean that the hydraulics driven by that engine are leaking, not just unpressurized (I'd have to go through a bunch of uncontained failures and see what else failed) -- and the Airbus manual stab/rudder control still depends on hyd pressure for the servos

@UnrecognizedFallingObject (first comment): IIRC, none of the flight controls on that plane were actually affected (they don't go through the roof, at least not on the 737).

But, is such a failure possible?

The answer to if such a failure is possible is on some level yes. But for what its worth no. There are a few things to consider.
Redundant and Independent systems: Aircraft typically have not only redundant systems but independent systems in other words not only are there back ups but they are often separated so that a failure in a single system or set of systems should remain isolated. From a power standpoint a plane gets is electrical power from the turbines driving a generator. Each turbine has at least one generator (maybe more Im not sure) so if a single engine were to cut out you would not lose power generation. You may need to shut the lights off to not over draw the generator but you could still power the key systems. Aircraft also have batteries in the case of total engine failure. keep in mind that in a total engine failure you will be on the ground long before you kill the batteries I would think.
Multiple Electronic Busses: To my knowledge larger airplanes have multiple electric busses each of with can be independently tied to and disconnected from the main buss as well as joined if needed. If one buss fails you can switch over to the other to draw power for the aircraft. Keep in mind that if a surge causes the failure the components would be protected by the fuses.
This is even finding its way into smaller GA airplanes. It is possible to order some small planes with 2 alternators now I believe the Cirrus SR-22 can be had in such a configuration.
Now that we have established that there are actually multiple central power connections lets look at why its so remote they fail. If you take 2 systems that are (for the purpose of this example) created equally, have the same run times on them and are serviced the same etc. they would have the same chance of failing at any given moment in time. That being said the chances of them both failing at the same point in time is so remote it can almost be ignored. The idea here is that we can accept a single system failure, complete the flight, and then send the plane in for repair.

There have been incidents where the primary flight displays have failed somewhat as you describe: example 1, example 2 (one each for Boeing and Airbus so I'm not taking sides). So yes, things can fail, causing cockpit displays to go blank and other problems.
That said, there's a lot of redundancy in these systems. Cockpit display systems are somewhat separate from flight control systems, so the failure of one may not impact the other. As
UnrecognizedFallingObject notes, there are even some options for manual control in the event of a total fly-by-wire failure.
Modern glass cockpit aircraft have a small display, usually toward the center, with standby instruments (Airbus calls it the Integrated Standby Instrument System). This is a separate LCD display, powered independently by the emergency bus and driven by its own computer, designed to display altitude, airspeed, and attitude. This provides the crew with essential information toward making an emergency landing.


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Are there any IFR-certificated and equipped aerobatic aircraft?
Are there any acrobatic-category aircraft that are also type-certificated/rated and equipped for flight under the instrument flight rules?  What limitations would be placed on such an airplane when flying as an IFR aircraft vs. a VFR aircraft?

How else are you supposed to practice flying the inverted ILS?

Yes: Consider the Zlin Z 242. The type certificate shows that it is certified in the Acrobatic, Utility, and Normal categories. In the section for Optional Equipment, the aircraft can be sold with

Communication and navigation equipment VFR DAY, VFR NIGHT OR IFR according to actual requirements of the customers

Generally, if the aircraft meets the minimum equipment requirements and inspections, and the POH, Type Certificate or other manufacturer's documents allow it, an aircraft may be flown IFR in IMC.
There would be no particular limitation on operations specifically because the aircraft is capable of aerobatics, but the pilot would still need to follow their clearance. This would significantly limit the benefit of the instrument certification. (Note that even if deviations in altitude and flight path were approved, aerobatics still wouldn't really be possible.)

The last paragraph doesn't make any sense.  Aerobatics and IFR flight are not compatible, but neither are they mutually exclusive.  An IFR capable Aerobatic plane would simply allow you to fly IFR to an airshow, and when the weather clears, perform aerobatics.  They are two very separate capabilities, neither of which limits the other.

Most combat trainers are equipped and rated for IFR.  For example, Pilatus makes the PC-21!.

Another example would be the PZL-130 "Orlik".


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Can pilots wear glasses/corrective lenses inside the cockpit?
I was wondering If I could wear corrective lenses/glasses inside the cockpit If I have a certified class 2 medical certificate?

Which country or regulations are you asking about?

Not only can they, but they must if it is required by their medical certificate.

Yes.  All current classes of FAA-issued medical certificates allow for the use of corrective lenses in the cockpit.  See Title 14 §67 Medical Standards and Certification.

According to the FAA, yes. For a Class 2 Medical:

Distant Vision: 20/20 or better in each eye separately, with or without correction.
Near Vision:  20/40 or better in each eye separately (Snellen equivalent), with or without correction, as measured at 16 inches.
Intermediate Vision:  20/40 or better in each eye separately (Snellen equivalent), with or without correction at age 50 and over, as measured at 32 inches.

Notice the "with or without correction".

According to the CAA in the UK,
Class 1
Refractive error

(1) At initial examination an applicant may be assessed as fit with:

  (i) hypermetropia not exceeding +5.0 dioptres;

  (ii) myopia not exceeding -6.0 dioptres;

  (iii) astigmatism not exceeding 2.0 dioptres;

  (iv) anisometropia not exceeding 2.0 dioptres;

provided that optimal correction has been considered and no significant pathology is demonstrated.
BUT there are a long list of exceptions and further conditions that may well disqualify you anyway.
And the requirements for a class 2 medical are stricter, add limits on top of this.

Hey - I'm not sure if it's the other way round in the USA, but in the UK Class 1 is the strict, commercial medical and Class 2 is what you need for a PPL and a lot less strict.


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What is the easiest way to update sectional charts with bulletin changes between printings?
I am a student pilot and just purchased my first sectional chart a couple months ago. While the chart itself is within the effective time frame, there are Aeronautical Chart Bulletins that contain updates to the printed chart. I am supposing that I should notate these changes on my paper chart so I have the most current information, but I am trying to find the best way to do this.
My initial thought was that I could use something like http://vfrmap.com/ to see what those changes look like on an actual chart, and notate them on my chart to match. -But the changes are not reflected on vfrmap, or any other online source that I have found.
Of course I could just create them manually, based on the information listed in the bulletin, but it would be a lot easier to copy them from an existing online chart.
For example, here is the change listed in the bulletin:

But here is the airport on the vfrmap.com:

Maybe I am going overboard here, but I would love some input from more experienced pilots on a practical way to keep my chart current between (6-month) printings.

...or any other online source that I have found.

Sites like VFR map, Foreflight and the such get the maps from the freely available digital downloads the FAA produces. These downloads coincide with the physical printed versions and update along with them.
The FAA (at least partially) explains what to do right on the link

...why we publish the Aeronautical Chart Bulletin in the back of
  each Airport/Facility Directory (AFD) every 56 days and why it is
  important that you consult the Notices to Airmen (NOTAMs) prior to
  each flight. FAA regulations state that a pilot must be familiar with
  all available information concerning a flight. This includes having
  up-to-date charts and keeping them current...

The main point here is to check the available supplements and make sure you have all the up to date information relevant to your flight.
But lets talk about this in practice a bit because its important to understand what changes and what you should mark. NOTAMS often inform you of things like VOR outages and runway closures. In some cases these are permanent but in many cases they are temporary due to construction, maintenance or calibration. You should not go crossing out VOR's or Airports on your chart if they are only going to be down for potentially the duration of your flight that day (as relevant to you). With that in mind you should notate them as appropriate to your flight.
In many cases if the the runway closure or VOR outage is not along your route or a nav aid you plan to use it is of no relevance for you for that flight anyway. When going VFR you may even hear the briefer (if you call in) ask "would you like to hear about nav aid outages in your area". You can easily respond no as you may not be using VOR's to navigate that day.
The key is to make sure you have all the information on your destination and your posible alternate fields along the route and any potential deviation plans. You don't need to keep the whole span of the chart up to date, thats why they reprint them.
If your flight school allows you may want to look into something like foreflight or garmin pilot which makes sure your stuff is always up to date.
FWIW when i used paper charts I used to write all over them and often replace them before they expired because they were written all over.


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When should I start turning in this circling approach?
I've been practicing circling approaches in Homestead General Aviation Airport in Homestead, Florida (X51). I performed the RNAV (GPS) RWY 10 approach with circling minimums.

My questions are:
a) For example if the active runway is Runway 18, then how exactly is the circling procedure performed? Should I start circling after crossing the MAP (XAGUH), or before?
b) And if the active runway would be 36, then the circling procedure would be to enter a downwind, then a base and then a final. Until when should I maintain the Circling MDA?

Charles, I think your questions are answered by this question:  https://aviation.stackexchange.com/questions/1828/when-flying-a-circling-approach-is-it-permitted-to-begin-the-visual-circling-se

The FAA instrument flying handbook discusses techniques and the procedures by which circling approaches must be conducted.1
You must remain within the protected area or circling radii, but as soon as you enter this area, you may begin the circling maneuver.  Usually, this means you will begin the maneuver well before the missed approach point.  In particular:
a) "The criteria for determining the pattern to be flown are based on personal flying capabilities and knowledge of the performance characteristics of the aircraft. In each instance, the pilot must consider all factors: airport design, ceiling and visibility, wind direction and velocity, final approach course alignment, distance from the final approach fix to the runway, and ATC instructions."
b) "Fly no lower than the circling minimums until positioned to make a final descent for a landing." (page 10-20 of Ref 1).


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What is the comparative roughness of fabric surfaces?
"The Design of the Aeroplane" by Darrol Stinton (1983) lists comparative roughness of aircraft skin material (pg 154). Unfortunately it does not list dope & fabric.
What is the comparative roughness of dope and fabric surfaces?

It varies with the level of finishing effort, and depends mostly on how much the weave of the fabric was filled in.  If the weave shows in the fabric, you can expect the surface roughness to be similar to lacquered wood.  If there are enough dope coats to fully fill the fabric, you'd have something similar to commercial metal painted finish.  But if you really go to town you can get to the filled and polished glass surface finish.  You'll see some award winning homebuilts at Oshkosh with a fabric finish that looks like shiny mylar plastic, but is actually regular fabric and dope.   Many many coats, and many many hours of sanding and rubbing.
Some of the latest finishing systems can produce that finish without zillions of coats and hours of rubbing.
This is all notwithstanding all the surface irregularities you have with fabric due to the use of reinforcing tapes etc., which may make the whole issue of surface smoothness moot anyway.

"It varies" is true for ALL finishes, but that should not prevent a generalized standard, should it? Wind tunnel tests are usually based on a "standard" roughness which is at least to some extent subjective.

Well if you're looking for a published engineering standard for fabric, I'm not aware of one and perhaps my answer is not really appropriate to your question.


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Has the United States started a drone crop spraying program for farmers?
On "NHK World" (2018.06.01), a news segment showed Chinese farmers being taught to use drones to spray chemicals on their crops. The Chinese Government hopes to train all the farmers in an entire province.
Has the United States started any similar drone crop spraying program for farmers?
Is there any speculation on impact to future agriculture pilot jobs?

Well if we're going to keep jobs by keeping people using obsolete tools, how about tilling the fields the old fashioned way, bonus, also makes jobs for the mules!


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Why doesn't GA aircraft use the JASC/ATA document standard?
By international law large aircraft are required to categorize aircraft information using a standard called the ATA document numbering system (Air Transport Association).
The ATA document chapters allow a pilot or mechanic to find the same information for any airplane under the same chapter/sub-chapter number. Thus a chapter number for a Boeing 747, B737, or Airbus 380 will be the same. Examples of this include Oxygen (Chapter 35), Electrical Power (Chapter 24) and Doors (Chapter 52).
A few examples of ATA chapter numbers...
ATA No. ATA Chapter name
ATA 00  GENERAL
ATA 01  MAINTENANCE POLICY
ATA 02  OPERATIONS
ATA 03  SUPPORT
ATA 04  AIRLINE USE Aircraft Handling
ATA 05  TIME LIMITS/MAINTENANCE CHECKS
ATA 06  DIMENSIONS AND AREAS
ATA 07  LIFTING AND SHORING
ATA 08  LEVELING AND WEIGHING
ATA 09  TOWING AND TAXIING
ATA 10  PARKING, MOORING, STORAGE AND RETURN TO SERVICE
ATA 11  PLACARDS AND MARKINGS
ATA 12  SERVICING
ATA 14  HARDWARE AND GENERAL TOOLS
ATA 18  VIBRATION AND NOISE ANALYSIS (HELICOPTER ONLY)

About 20years ago the FAA strongly urged GA aircraft to follow the standard with the anticipation that it would soon become law - it never did.
Why has the International community not mandated GA aircraft use the JASC/ATA document standard.

Would seem the obvious answer is because most GA aircraft were built, and their documentation written, quite a bit longer than 20 years ago.

Are you suggesting rewriting old manuals?

Newer GA aircraft do generally follow this format like the SR22 service manual. As jamesqf mentioned in the comments many of the popular GA aircraft were designed upwards of 50 years ago and their airworthiness and maintenance procedures have not really changed a whole lot thus the manuals are sill what they were when published 50 years ago.
14 CFR § 23.1529 requires that instructions be drafted (or in the process of being drafted) during certification. Like anything in aviation once its certified, why rock the boat by re-writing the manual just to comply with a format.
More reading...

You reference Part 29.1529, but that Part applies to Transport Category Rotorcraft. GA airplanes are covered by Part 23.  In recent years Part 23 has been simplified to eliminate implementation details. 14 CFR 23.1529 specifies the instructions must be in accordance with Appendix A of Part 23. It does not require anything approaching ATA standards, simply stating;
(b) The format of the manual or manuals must provide for a practical arrangement.

@Gerry you are correct i grabbed the wrong one (they are all linked in 8110.54A). I never stated it requires any standard but merely that the docs are required and there would be no reason for a company that already had an approved document (an expensive journey behind) to update to a non required standard.


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What instruments were "standard" in WWI aircraft?
What instruments made up a typical panel of a WWI airplane?
Did any country regulate the required cockpit instruments prior to the United States "Air Commerce Act" of 1926?

Not really an answer, but I've seen some WW-I/pre-war aircraft that had nothing more than a spring-loaded needle moved by wind to indicate airspeed. The fancier ones had colored regions to indicate danger zones.

Even more primitive (but not in the cockpit) are telltale yarns in the guy wires (like sailors used before aeroplanes).

Gliders still use yarn taped to the nose or windscreen as a slip indicator. When I was learning to fly in 1975, my instructor put yarn on the windscreen for a couple flights when I started doing (uncoordinated) ground reference maneuvers (it doesn't take long to learn how much rudder to use instinctively).

Eyes and ears(including the inner ear) were the only "standard" instruments at that time. Although some crude gauges were in moderate use.(not all were found in all aircraft and some aircraft had few or none) DaveK listed most of them. A map and some navigation tools would also be fairly common, especially on longer flights. Fuel gauges were mostly of the sight glass type. (clear tube to physically see the fuel level)

If you are asking about military aircraft there would often have been a compass, oil pressure gauge, a not very sensitive altimeter, a tachometer and some sort of airspeed indicator. Not sure about fuel gauges. I have also seen bank indicators. There was not the standards for instruments as in later days and some items were specific to individual aircraft types. Some of these guys could fly a sort of instrument flight but had not much in the way of navigation aids.


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How does dropping ordnance affect the weight and balance of the aircraft?
During a mission, military aircraft often shed a lot of weight from takeoff to landing.
An F-15C (empty weight: 28,000 lbs) carries an internal fuel load of 13,455 lbs (almost a third of take-off weight not including pilot and ordnance).
A cargo plane like the C-17 may perform a combat off-load while airborne.
Meanwhile, the aircraft I worked on in the army were deadlined whenever 1% of total aircraft weight was removed, requiring either the part to be reinstalled or a total reweigh prior to next flight.
Operationally speaking, how are these "fat burning exercises" completed without jeopardizing crew and equipment due to shifting weight and/or CG?

It sounds like the concern with removing a lot of equipment was that the pilots might not know what the weight and balance are. They know how much cargo and fuel they're carrying, and where in the plane it is, so they (or a computer) can do weight and balance calculations.

There's a number of ways they can do it - burning fuel from certain tanks first to offset it, loading the cargo in such a way as to not affect it much. Keep in mind that a near-empty cargo plane will very rarely be out of CG - it would be primarily in the middle of exercises - i.e. with half the payload dropped, and half still on board - that it would be an issue

dropping a 2,000 lb bomb i would think has more than a small effect to handling.  but i'm not a fighter pilot, so curious.  (maybe it just makes it more maneuverable?)

my initial thoughts for the f-15 is that fuel and weapons would have to be stored at cg.  that way, there is no shift fore/aft when fuel is consumed or ordnance dropped.  c-17 loadmasters, on the other hand...

Look at any dedicated bomber, and you'll see that the bomb bay is placed at the center of gravity.  This question is really only interesting for aircraft that weren't designed from the beginning as bombers.

@mark my thoughts exactly, especially with that observation.

This accident http://en.wikipedia.org/wiki/Air_Midwest_Flight_5481 seems relevant...

the aircraft I worked on in the army were deadlined whenever 1% of total aircraft weight was removed, requiring either the part to be reinstalled or a total reweigh prior to next flight.

That's for internal equipment, which will affect the empty CG data.
All aircraft lose weight during a trip. Aircraft designed to lose a lot of weight all of a sudden ( C-5 airdrops, B-52 carpet bombing ) have the details and limits calculated during the design. In the case of bombers, the ordinance will be hanging near the wings and will have a specific release order, usually back to front.
A tailgate aircraft dropping something will also have very specific procedures based on the weight of the pallet going out the back door. If it's delivering pizzas the procedure is along the lines of "whenever, dude". If it's a tank, it has to be positioned and locked down until the moment of release.
The above is very noticeable by the flight crew.
If the load stays (or gets stuck) on the back ramp, you're usually done. I used to skydive - 10 people on the back end of a smaller tailgate like a Casa can and do cause the plane to stall. The rules were stay at the front until it's your turn, then position and exit in a prompt manner. If the buzzer goes, let go NOW. Unlike a cargo plane, jump planes have 12,000 feet to recover from the stall.

your second paragraph nailed most of the information i was looking for.

The reason for the traditional WWII and earlier "bombs away" call from the bombardier was to warn the crew that the aircraft was going to popup suddenly. If crew weren't paying attention they could get smacked by or smack down onto something.

If the load stays (or gets stuck) on the back ramp, you're usually done. "Done" as in "toast", i.e. the plane will inevitably stall and crash? How much time (how many seconds) should it take to shift a tank from the centre of gravity to out the back door?

@ChrisW yes, as in a quick and firey demise for all aboard. Drag-chute extraction if heavy cargo (like a tank) is very fast - they are often done at mere meters above the ground. Inertia takes care of the brief out-of-limits balance issue

@ChrisW It would look something like this: https://www.youtube.com/watch?v=imvbv5KdD44 (it's believed a cargo strap came loose or broke and weight shifted dramatically and rapidly).

First, let's agree that when we say CG we're talking about the longitudinal CG, not the lateral or vertical CG.
Every aircraft has a basic operating weight, typically abbreviated b.o.w. or just BOW. The BOW is the empty weight of the aircraft + the required crew + undrainable fluids. For this BOW there is a known CG. The BOW and it's CG are the base values from which further calculations proceed. If you add or remove a part of the aircraft, you have changed the BOW and its CG, and even though the change may be small, it's important to recalculate the new BOW and its CG because you now have a new baseline. If you later load the aircraft right up to a weight limit or against a CG limit but use an old baseline rather than the new one, you may exceed weight or CG limits or both.
So now you load the aircraft: passengers, cargo, fuel, armaments, whatever. Let's say one of the items is the 2,000 lb bomb referred to in a previous comment. If that bomb happens to be right on the CG, when you release it your weight drops by 2,000 lbs, but your CG will not have changed. If, say, the CG of the bomb was 10" forward of the CG of the aircraft + the bomb, when you release the bomb you will change the CG moment by 20,000 inch-lbs. The usual convention is that forward is negative, aft is positive, so you would have incurred a -20,000 inch-lbs movement. As long as the resulting CG is still within the forward and aft limits, all is well, but you'll need to trim out your increased pitch because your CG has moved aft.
Note that if your bomb was only 200 lbs but it was 100" forward of the existing CG, you would get the same moment change since 10 x 2,000 = 100 x 200.
Let's say you're doing an airborne offload of a large truck out of a C-17. Further, let's say that just prior to the drop your CG is close to the aft CG limit for normal sustained flight. When you drop the truck it has to move to the rear before going out the back, but that happens quickly and you have enough airspeed that the elevator can counter the momentary movement of the CG past the normal aft limit for sustained flight. If the truck should become stuck just before dropping, you'd have a serious problem. See the Bagram 747 video for what happens when a vehicle shift in flight puts the CG way aft of the limit.

It really depends on the aircraft. Your F-15C as an air superiority fighter would primarily carry missiles; it can carry four AMRAAMs or Sparrows underbelly (the Sparrow's been all but phased out of the arsenal, but was a common loadout in the F-15's heyday) and four Sidewinders on the sides of the wing hardpoints, with either an additional pair of AMRAAMs or Sparrows on the bottom of the hardpoints or two 600-gallon droptanks (useful for extended CAP missions). That's in addition to 650 rounds of Vulcan 20mm.
These missiles are fairly lightweight; each AMRAAM only weighs about 340 lbs (compare to 500, 1000, 2000 or even 5000-lb bombs), to total no more than about 3500 lbs total ordnance weight when loaded for a "scramble interceptor" profile (get in the air quickly, target the enemy and if they close within X miles, fire at will). It's counterproductive for a plane designed to dogfight to be hampered by heavy munitions (when the F-15 was first designed, the aim was a pure air superiority fighter to combat the perceived threat of the MiG-25, and the mantra was "not a pound for air to ground"). Thus, the primary change in mass and balance during a mission is fuel. If the loadout includes droptanks, they'll be consumed first, as they can be jettisoned when empty (or if an aerial threat closes to dogfighting range or a missile is launched) and even if kept for re-use, draining them first increases the aircraft's roll rate faster in preparation for mixing it up.
The F-15E ground strike variant is a similar airframe with a very different mentality; the aircraft is able to lift a whopping 49,000 lbs of combined fuel and ordnance. It was designed to replace the F-111 Aardvark as a light penetration fighter-bomber for long-range precision strikes on hardened targets using large laser-guided bombs (missions that generally call for a "one pass and haul ass" mentality by the crew), but the Strike Eagle has also been used for SEAD, tank-plinking and other missions generally requiring more total "drops" and thus a more gradual change in aircraft weight and balance.
The performance and handling of the aircraft during ingress versus egress is usually totally different. Enroute, a plane full of fuel and ordnance is a slow, lazy-feeling thing that's really hard to maneuver. By the time you make your approach, though, you've burned over half your fuel, and even with the heaviest bombs this thing can take, fuel is still the majority of the aircraft's max take-off weight. When dropping Mk-82s or laser-guided variants, they typically have to be dropped in pairs; the plane just isn't stable with a one-ton imbalance between the wings (I can't think of many aircraft its size that would be). 500-lb Mk-84s and GBU-12s are still typically dropped in pairs, but it's possible to drop one at a time and keep a stable airframe left to right. The fire control computer on most planes with multiple front-to-back hardpoints, including the F-15, will typically release ordnance front to back, bottom to top, reducing the possibility of the ordnance getting tangled up with others on the way off the aircraft or to the ground (it also helps improve accuracy to use the front stores first as airflow over the weapon is cleaner with nothing in front of it). To maintain weight balance front to back, it is possible to configure the FCC to release from the ends toward the center, but it only matters at all for the F-15E's CFT hardpoints which are in three groups from front to back; it's a bigger deal for a heavy bomber like the B-52, which can carry on external hardpoints running practically the entire length of the fuselage.
Either way, once the ordnance is gone, the F-15E feels like a completely different plane, much more like its air-superiority cousin. The CFTs (and the added targeting pods and the Wizzo's seat) make the F-15E inferior in a dogfight to the C-variant, but it stands a lot more of a chance with a similar loadout versus having almost 15,000 pounds of bombs on the rails.

The airplane is going to have established weight and CG limits already established (unless you are asking about test planes, which I feel would be an entirely different question and answer).
That being said, there are very quick methods for calculating weight and balance during flight for both shifting weight, and dropping weight.  On my NIFA (National Intercollegate Flight Association) flight team we practice these type of formulas all the time.  Using a CR (or even E-6B) you can follow these formulas to find what the CG will be after dropping a known weight from a known arm.  Any pilot will know the amount and location of the payload or "ordinance" before dropping it, as well as running these calculations.
Here's some note cards that I have.  On here you can see the formulas for weight shifted (2d down on the right) as well as weight added or removed (bottom left).  If you have any questions about these type of calculations feel free to ask.

i don't quite understand this notation... is there an equals sign between the two sides? a simple formula would make better sense.

Think of the top (over the line) as the outer ring of the CR, and the bottom (under the line) as the inner ring of the CR. For the weight shifted equation, you would line up the the amount of weight shifted (on the outer scale) over the total airplane weight (on the inner scale).   Then you would look over the distance the weight was shifted to find the change in the CG.


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What type of aircraft uses this long flashing pattern?
5:00 am every morning I went to Rice-Field Road for a jog. I always see airplanes with flashing lights. All of them have the same flashing patterns except for the one that I just saw yesterday.
The pattern is a long flash. If I'm not mistaken (I counted it), it flash slowly for more or less 10 seconds and then gone at same count.
I was curious and surprised that there's an aircraft that has that kind of flashing patterns. I gazed at it for a minute until it was out of my sight.
I also found this and I'm not satisfied since I'm more concerned of what type of aircraft was it.
What type of aircraft uses this long flashing pattern? References might also help. Thanks in advance :)
UPDATED:
This was the exact location where I saw it.
I'm not so familiar how high or low an aircraft might be for a landing. But I guess the aircraft was too high for a landing and its direction was straight away from a landing area. And yeah, the flash was color white.

10 seconds is not a flash. Could you have seen the landing lights? They are very bright when the plane is flying almost directly towards you, but not visible from other angles.

Where is "Rice-Field Road"?

Could the aircraft have been passing through clouds that obscured the lights for a period of time?

@Notts90, that's what I'm thinking before but the sky was clear that time

I'm assuming your question is : "What type of aircraft was it?".
Actually, we can't answer your question but you could try using Flightradar to identify the aircraft by watching a playback of the regarding time and place.
To hand you some trivia: If it was a red flashing light (the beacon) it is totally normal that the kind of flash varies. Modern aircraft use 360 degrees flashing lights (depending on the manufacturer it flashes faster or slower). Older (and most small aircraft) use rotating beacons which is just a red light shining in one direction which is turned by an motor (turns faster or slower depending on the motor).
If it was a big white light it could possibly have been the landing lights (which flash only on some aircraft) or the strobe light in the wings outer edges. They usually flash 2 times in one second and repeat the two flashes after 1 second of pause.

IIRC the strobes must flash every second. It may be two flashes very short after each other, but they would repeat every second. 3-4 seconds would be incredibly sparse.

@JanHudec There are some LED strobes that flash a lot slower than the old-style xenon tubes we're used to, but almost certainly not once every 3 seconds (IIRC the FARs specify 40-100 flashes per minute).

People tend to mention the colour of lights that aren't white, so I'd assume a white light. But still good to cover the other options.